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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
tobias wrote ...
Thanks PM for educating me on the NOS subject. If it is to be a used one then I prefer getting the whole piece of used equipment instead of just the valve :)
Ebay sellers who use valve testers - unfortunately a minority -generally put the tested anode current of used valves in the sales listing, which seems to me to be good practice.
Unless a filament or heater has failed, valve testing isn't just a pass/fail question like light bulbs. As thermionic valves grow old in their service life, the cathode emission falls off bit by bit until it is insufficient to maintain the anode current, and distortion will result as the linearity of the grid voltage/anode current characteristic curve is bent out of shape. (These graphs are shown in the valve's data sheet) The decision to replace a valve with reduced emission will depend on the overall spec of the equipment in which the valve is being used, and the judgment of the service engineer, or a company policy "All valves will be replaced when they have lost 10% emission."
A valve that had fallen below spec for a military or aeronautical radio might still have lasted for several years in a piece of consumer audio equipment
You can test the basic function of triodes without a valve tester by applying heater current and measuring the anode current when you have so many volts on the anode, and so many minus volts on the control grid (often written g1) and comparing the test result with the characteristic curve in the data sheet. With tetrodes it is just the same, but you must additionally supply current to the screen grid (g2) at the voltage given in the data sheet. To test pentodes, as for tetrodes, but with the suppressor grid (g3) connected to the cathode (k). By the time you get to testing triode-heptodes you will have a whole spaghetti of flying leads and meters...
Is there not a way to wire a pentode for triode operation? I thought I remembered seeing a schematic where one of the three grids was tied to anode, one to cathode, and one for signal input like in a triode.
I've got some beam power pentodes that I'd been considering making a simple two valve amp out of. I know I biased one into linear region and did a quick class A amp test using a silicon preamp a few months back, but I didn't really know what I was doing as far as using the grids correctly. It was after the test that I read the bit about a triode mode pentode. It was a pull from a dead transmitter, so I wanted to see if there was any life at all basically.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Sigurthr wrote ...
Thanks for that info, PM!
Is there not a way to wire a pentode for triode operation? I thought I remembered seeing a schematic where one of the three grids was tied to anode, one to cathode, and one for signal input like in a triode.
I've got some beam power pentodes that I'd been considering making a simple two valve amp out of. I know I biased one into linear region and did a quick class A amp test using a silicon preamp a few months back, but I didn't really know what I was doing as far as using the grids correctly. It was after the test that I read the bit about a triode mode pentode. It was a pull from a dead transmitter, so I wanted to see if there was any life at all basically.
Some pentodes were designed so that they could also be operated as triodes, and their data sheets give details of the valve parameters in triode mode.
The 6BS7 discovered by Ash in his shed is just such a valve. Note that triode connection in this case means connecting the screen grid (g2) to the anode and the suppressor grid (g3) to the cathode.
You could, I suppose, configure any pentode as a triode in this way, with all the loss of gain and likelyhood of instability that this implies, but you would have to discover the new parameters and characteristic curves yourself. The correct bias voltage has to be applied to the control grid if a valve is to work in Class A and enjoy low distortion..
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
New Old Stock
Many 'new old stock' valves offered on ebay are in fact used valves put in a new box.
A genuine 'new old stock' valve of the 1950s and 60s will still have its wrappings inside the box - usually a combination of tissue paper and single thickness cardboard wound round the valve a few times, and often held in place with an elastic band which may have perished with age, but should still be in the box. Always included will be the original guarantee certificate. Look for valves where the paper seal on the box has not been broken.
Genuine NOS Soviet-era valves ALWAYS have a quality control certificate and a data sheet wrapped round the valve and held in place by an elastic band. The paper of the documents is always yellow with age, and may be brittle.
The numbers and letters printed on the glass of a genuine 'new old stock' valve will be sharp and crisp, with no signs of wear.
Types of USED old stock frequently offered as 'new old stock' on ebay:
Valves in new plain white boxes with no wrapping or documents inside the box. (You can buy empty valve boxes on ebay too!).
Valves in genuine old boxes, but in an open condition without wrappings and guarantee inside. These valves are often valves that have been removed from equipment by service engineers because of reduced emission or other faults. It was part of the accounting system of that time for the service engineer to put the bad valve back in the box that the new valve came in, so the managers could see that the engineer had replaced the valve, and not stolen it. They were expensive items. Bad valves like these in genuine boxes may be sold by an innocent person who doesn't know the practice of putting the failed valve back in the box. Many of these valves will work to some extent, and buyers may not notice that the valve has reduced emission or an intermittant fault.
Some valves that are said to be 'tested' by ebay sellers have only had the filament or heater continuity tested.
Always ask an ebay seller why a box has been opened, why it is plain white, why is there no maker's guarantee inside the box, what tests have been performed etc.
Hmm. I've just come by a large box of audio valves, and was wondering whether to loose them onto eBay, like, should I test them first? I guess the answer is yes, otherwise I could get a bad name. What would sufficient tests be? Ianode at zero cathode-grid volts should be cathode emission limited, and be more sensitive to that parameter than with a finite grid-cathoe voltage, as long as it's listed as a parameter. I presume that curves for most valves can be found on the net, for the commonly used audio ones at least?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Would it not be possible to build a 'headphone amp' around a single valve/vacuum tube, of the type normally used for the pre-amp, like, for example, a 6BS7 or similar high gain miniature tube?
You don't need the power valve for headphones, do you?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Dr. Slack wrote ...
Hmm. I've just come by a large box of audio valves, and was wondering whether to loose them onto eBay, like, should I test them first? I guess the answer is yes, otherwise I could get a bad name. What would sufficient tests be? Ianode at zero cathode-grid volts should be cathode emission limited, and be more sensitive to that parameter than with a finite grid-cathoe voltage, as long as it's listed as a parameter. I presume that curves for most valves can be found on the net, for the commonly used audio ones at least?
I will use the famously wonderful double triode 6SN7 as my example.
Go to the 6SN7 datasheet here: (The "GT" on the end of 6SN7GT means it is a 6SN7 with "Glass Tube" encapsulation.
Scroll down to the first graph which plots plate current against plate volts for a range of grid voltages from Ec = 0 to Ec = -24V.
Connect your 6.3V heater current and allow valve to warm up for 60 seconds or more.
Apply whatever HT+ voltage you have to hand - say 200V - to the anode of the first triode and HT- and Earth/Chassis to the Cathode.
Apply a negative voltage - say -10V to the grid and the + grid volts to Earth/Chassis.
Now looking at the graph, we see that for an anode voltage of 200V, and a grid voltage of -10V, we should have an anode current of about 1 mA. There is no digital precision with this, and ± 10% should be OK for non-critical applications.
Test the curve at two other points and if it matches the datsheet fairly well, you can declare the valve OK, with a reasonable expectation of accuracy.
Flick the valve with your finger, or gently tap it with a screwdriver etc, while watching the anode current. It should not fluctuate.
With tetrodes you'll have to supply the screen grid (g2) with the voltage specified in the datasheet and then do the same test using the control grid as for the triode.
With pentodes, as for tetrodes, but with the suppressor grid (g3) connected to the cathode (k).
There are, of course, more advanced and sophisticated tests, but these simple procedures will eliminate the great majority of bad valves.
The absolutely best thermionic valve datasheet collection on the web is Frank's Electron Tube Datasheets which is free, has no ads, and can be found here: The search screen button is at the top right.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Would it not be possible to build a 'headphone amp' around a single valve/vacuum tube, of the type normally used for the pre-amp, like, for example, a 6BS7 or similar high gain miniature tube?
You don't need the power valve for headphones, do you?
I would say it would be possible, though you will still need a matching transformer to match the anode impedance to the headphones. You won't get a lot of volume, but it should be adequate for ordinary listening.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
This afternoon I found a small 100V audio line transformer in the shed, and did some simple tests to assess its suitability for use as a valve audio output transformer.
The primary had six lugs (the two ends of the winding, with four taps along it) and the secondary two.
The bad part of this is the inductance of the primary between taps 1 and 4, which is only 5H. Farewell deep bass response! I could take an interest in flute music I suppose.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
This afternoon I found a small 100V audio line transformer in the shed, and did some simple tests to assess its suitability for use as a valve audio output transformer.
The primary had six lugs (the two ends of the winding, with four taps along it) and the secondary two.
The bad part of this is the inductance of the primary between taps 1 and 4, which is only 5H. Farewell deep bass response! I could take an interest in flute music I suppose.
I was thinking that you 'may' be able to use a 240-12 volt step-down transformer, with a turns ratio ~20:1. Some have thinner laminations than others (required for reducing eddy currents at the higher frequencies).
I have a few, but none with particularly thin laminates. I've not done any measuring yet, though.
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Headphone Tube Amplifier
